Imaging devices form images on image receiving members that include paper and other print media. Different imaging or printing techniques, which include laser printing, inkjet printing, offset printing, dye-sublimation printing, thermal printing, and the like, may be used to produce printed documents. In particular, inkjet imaging devices eject liquid ink from printheads to form images on an image receiving member. The printheads include a plurality of inkjets that are arranged in some type of array. Each inkjet has a thermal, piezoelectric, or mechanical actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The printhead actuators respond to the firing signals by ejecting ink drops onto an image receiving member to form an ink image that corresponds to the digital image used to generate the firing signals. The size of the ink drops and the timing of the ejection of the ink drops are affected by the frequency, wave shape, and amplitude of the firing signals.
In an inkjet printer, the digital image data often specify different areas of an image that receive different amounts of ink during the imaging process. A “halftone percentage” describes the coverage of ink over an area of the image receiving member. For example, a 100% halftone refers to a high coverage area of the image that is fully covered in ink, such as a solid region of a picture or to dark text letters printed on the image receiving member. A low coverage halftone area refers to areas of the image where the ink covers less than fifty percent of an area, while the remainder of the area is the bare image receiving member. For example, in a 25% halftone area, the printer covers 25% of a given area of the image receiving member with ink drops and the remaining 75% of the area is bare. In a common configuration where ink images are formed on white paper, the drops in the halftoned area are spaced apart in a “dithered” pattern so that the human eye perceives light reflected from the ink drops and the paper as a uniform color that is lighter than a 100% coverage area. For example, a 100% black coverage area appears as a solid black color, while a 25% halftoned area using black ink appears as a shade of gray. The printer forms images with a wide range of halftones to form ink images, and multi-color printers can print different colors of inks in various halftone patterns to form images with a wide range of perceived colors.
Existing inkjet printers face challenges when printing images that include high coverage, high ink density areas and low coverage, low ink density areas in a single printed page. Specifically, larger ink drops are used in the high coverage areas of the printed images to provide a uniform coating of ink over the high coverage areas of the image receiving member. However, use of larger ink drops for low to moderate ink densities produces images with objectionable “graininess,” meaning the individual drops are more visible to the human eye. In low coverage areas, smaller ink drops are ejected to produce more uniform colors with a reduced “graininess” where the human eye perceives a blended color of the ink and underlying paper instead of the individual ink drops. While some existing printhead designs can be selectively configured to print either large ink drops or small ink drops on a drop by drop basis, such is not the case with all printhead designs. In many printhead designs that enable dynamic changing of the ink drop size, printing is more costly than printing with non-dynamic drop changing designs, which enable only a single ink drop size at a time. The additional expense arises from the time consumed in reconfiguring the printhead between a large ink drop mode and small ink drop mode and the inability of the printheads to change the mode of operation on an inkjet by inkjet basis within a printhead. In one known reconfiguration method, an image receiving member is moved past a printhead two or more times and the printhead prints ink drops of a selected size during each pass of the image receiving member. While this technique enables printing with different sizes of ink drops in a single image, the time required for multiple passes with the printhead over the image receiving member lowers the throughput of the printer. Consequently, improvements to inkjet printers that enable non-dynamic printhead designs to eject multiple ink drop sizes in an efficient manner would be beneficial.